X-ray diffraction apparatus for use with radioactive materials



Dec. 5, .1950 2,532,810

- DFHARKER X-RAY DIFFRACTION APPARATUS FOR uss wrm RADIOACTIVE MATERIALS v Filed Jan. 13, 1950 Inventor: David Hawker:-

by A M His Attor'neg Patented Dec. 5, 1950 UNITED STATES PATENT OFFICE .X-qRAY DIFERAOTION APPARATUS FOR USE WITH RADIOACTIVE MATERIALS David Harker, Schenectady, N. Y., assignor to =General1tElectric Company, a. corporation of New York Application January 13,1950, SeriaIZNo. 138,484

either-radioactive themselves or capable of emitting fluorescent radiations when subjected to X-rays.

In-the study of atomic arrangements of materials, the use of diffraction patterns caused by 'X-rays is well known and has given much information concerning the natures of unknown materials, as well as-the -structural arrangements of knownma-terials. From a correlation of the wavelength of'the X-rays employed and the diffraction patterns-formedwhen the material under-study is irradiated with such X-rays, valuable information can be obtained with respect tothe particle size of the-material, the state of strain-and other physical conditions and, in the case of unknown specimens, material can be iden- 'tified. :However, when the material being studied is itself radioactive, or isof the type which emits radiations when irradiated by X-rays, it is impossible to obtain accurate diffraction patterns in the usual X-ray spectrometers employed, since the radiations emanating from the material enter the radiation detecting apparatus and, too frequently, completely submerge the diffraction pat- :ternproduced by the :X-radiation. Accordingly, it is anobjectof my present invention to provide newend improved X-ra diffraction apparatus which is particularly useful for obtaining diffraction patterns of radioactive materials and materials capable of emitting fluorescent-radiations when subjectedto X-rays.

In its broadest aspect, my invention employs a curved crystalline substance-to rotate the converging diffraction beam from a specimen irradiated with X-rays through an angle such that radioactive radiation from the specimen cannot enter a radiation detector positioned subst ntial- 1y at the focal point of the converging diffraction beam. Additionally, radiation absorbent material ,is positioned between the material being studied and the radiation detector to eliminate all possibility of radioactiveor fluorescent radiations from the material entering the radiation-detectinapparatus to produce spurious sig- :nals.

fihe featuresof my invention which I believe ,to be novel are set forth with particularit in l the appended claims. My invention itself, however, both as to its organization and method of operation, together with further objects andadvantages thereof, may best be understood by reference to the'following descri tion-taken in connection with the accompanying drawinginwhich Fig. 1 diagrammatically illustrates the diffraction.

crystal 19. Preferably, crystal Ill apparatus of my invention, and Fig. 2 illustrates certain characteristics of the apparatus of Fig. 1.

Referring to the drawing, I have shown m diffraction apparatus as comprising a source of X-rays which may be a conventional X-ray tube I having an electron emitting filament 2 and a target electrode 3, the electrodes of the X-ray tube l having conventional connections to suitable sources of energizing potentials and control apparatus (not shown). The beam of X-rays '4 impinges upon a specimen 5 to be studied. The specimen 5 is supported in any suitable holdor 6 arranged to support the specimen 5 at such an angle to the X-ray beam 4 as insures focussing of diffracted X-ray beam 1 at point 8.

The specimen 5 may be of a substance capable of emittingradioactive radiations either'per se or through the interaction of the X-rays with the individual atoms of the material. Thus, the specimen 5 may comprise, for example, radium or radium compounds, materials which have been made radioactive by exposure to other radioactive materials, or-materials which become a source of X-radiation due to fluorescence when-irradiated with X-rays of the beam 4. In the latter category are, for example, com ounds of iron or cobalt which are exposed-to the X-radiation of a copper target X-ray tube.

The X-ray beam 4, after irradiating the specimen 5, forms a diiiracted beam l which converges or has itsfocalpoint at the narrow entrance slit 8 of a radiation detecting device 9 shown in dotted line. The device 9, which is used in a conventiona1 X-ray spectrophotometer for measuring the intensity of the diffracted rays, may be for example a Geiger counter, an ionization chamber, or a photoelectric multiplier device. Where the specimen 5 is itself radioactive, or is [capable of emitting flu-orescent radiations when subjected to X- rays, such radioactive radiations also enter the slit 8 along with the d sired diffracted beam and produce an undesired background pattern which maybe of agreater intensity than that of the desired-diffraction pattern and, in many cases, completely submerges the desired pattern.

In order to permit the study of radioactive and similar materialsbymeans of X-ray diffraction apparatus, in accordance with my invention, I place a crystal 19 in the path of the diffracted beam '1 to change the direction of the diffracted beam by an angle H of approximatel The crystal= l :1 may be formed of any crystalline substance of a type which'can be bent without breaking up its crystallinity, such as for example mica,

uuartz, or sapphire, and the exact value of the angle H depends on the substance comprising is relatively thin, having a thickness in the order of 0.002 inch or less, and has a fixed curvature. The crystal ii], positioned in the path of the diffracted beam, rotates beam 7 from its normal position to a new position 7, the rotated beam I converging substantially at a point l2. Accordingly, at point I2 is located the entrance of a radiation detecting device 9 which may be the same type of device as device 9 and, in my apparatus, replaces device 9. In order to insure that all rays of the diffracted beam I are focussed at point I2, crystal in has a fixed curvature. The curved crystal i must be tangent to an imaginary circle through points 8, E2 and the intersection of ray 1 with crystal I0, and must be curved so that its radius of curvature is equal to the diameter of the imaginary circle just mentioned. Depending on the nature of crystal I0, and the size of the angle i, one or another of the monochromatic spectral components of the diffracted X-ray beam "I is bent through angle 3 I by crystal I!) to converge at point I2. It is, therefore, possible to arrange matters so that a single wave-length of diffracted radiation only is converged at point I2, while all undesired radiation emanating from specimen (whether diffracted, radioactive or fluorescent) is not recorded by the detector 9'.

In its new position, detector 9 is removed from a direct line of travel of undesired radioactive radiations from the specimen 5. To insure that all undesired radioactive radiations from the specimen 5, including scattered and fluorescent radiations due to the impingement of beam 4 upon the specimen, are prevented from reaching detector 9', a radiation absorbent material, such as for example a lead block I3, is positioned be tween holder 6 and radiation detecting device 9.

In Fig. 2, crystal I0 is shown in enlarged View lying upon a circle having its center at a point I 3 and being tangent at point I5 to a circle I6 having a diameter lying along the line I 4I5 and equal to the distance between these points. In crystal I0, there are lattice planes parallel to the layers of atoms forming the crystal. Such planes are perpendicular to the plane of the drawing in Fig. 2 and are spaced apart a distance (1. These planes likewise form an angle 0: with planes normal to the crystal. According to the Well-known Bragg law \=2cZ sin I9) where x is the wavelength of the impinging rays, the lattice planes reflect incident X-radiation strik ing the planes at the crystal at an angle 0. The reflected rays, in turn, are converged to a point l2. As a result, rays of the beam 7, which normally would converge at point 8, are reflected by the crystal to converge at point I2. The points 8, l2, I4, 95 all lie on circle I6 which is tangent to the curved crystal at point l5 and which has a radius equal to one-half of the radius of the crystal. Depending upon the nature of the crystal and the wavelength of the incident radiation, the angles a and 0 have different values. In the construction of my improved X-ray diffraction apparatus, therefore, the exact position of detector 9' is determined by the nature of the crystal and the center of curvature of the crystal, these factors determining the angle through which beam 7 is rotated by the crystal. Preferably, this angle is relatively large and of the order of 90, the exact value depending upon the particular crystalline substance employed. Accordingly, entrance l2 to detector 9' is located at the focal point of the rotated beam I.

While I have shown a particular embodiment of my invention, it will of course be understood that I do not wish to be limited thereto since various modifications may be made and I contemplate by the appended claims to cover any such modifications as fall within the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. Apparatus for studying the diffraction pattern of a material capable of emitting radioactive radiations, either spontaneously or when subjected to X-rays comprising means for producing a beam of rays, means for supporting a material to be studied in the path of said beam and inclined at an angle to the axis thereof, a crystal supported in the path of the beam diffracted from the material, said crystal having a fixed curvature and being inclined at an angle to the axis of the beam sufiicient to rotate the path of the difiracted beam through a large angle, radiation detecting means positioned to receive the rotated beam, and radiation absorbent material positioned between the material to be studied and said radiation detecting means.

2. Apparatus for studying the diffraction pattern of a radioactive material comprising means for producing a beam of X-rays, a support for radioactive material to be studied positioned in the path of said beam, said support being inclined at an angle to the beam to form a converging beam of difiracted rays, a crystal having a fixed curvature mounted in the path of the diffracted beam, said crystal being inclined at an angle to the path of the diffracted beam to alter the path thereof by substantially an X-ray detector positioned to receive the altered beam, and X-ray absorbing material positioned between said support and said X-ray detector.

3. Apparatus for studying materials capable of emitting radioactive radiations when subjected to X-rays comprising means for producing a beam of X-rays, means for supporting a material to be studied in the path of said beam, said support being inclined at an angle to the axis of the beam, a crystal having a fixed curvature supported in the path of the beam diffracted from the material, said crystal being inclined at an angle to said diffracted beam sufficient to rotate the path of the diffracted beam by substantially 90, radiation detecting means positioned to receive the rotated beam, and radiation absorbent material positioned between the material to be studied and said radiation detecting means, said crystal being tangent to a circle passing through said radiation detecting means and the focal point of the diffracted beam, and having its radius of curvature equal to twice the radius of said circle.

DAVID HARKER.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,428,796 Friedman Oct. 14, 1947 2,452,045 Friedman Oct. 26, 1943 2,474,835 Friedman July 5, 1949 OTHER REFERENCES A High Resolving Power, Curved-Crystal Focussing Spectrometer for Short Wave-Length X- Rays and Gamma Rays, by J. W. Du Mond, Review of Scientific Instruments, Sept. 1947. 

